1. Field
The present specification relates generally to modems and facsimile apparatuses which execute G3 and G4 facsimile communications through telephone and ISDN lines, and in particular to such modems and facsimile apparatuses which employ a silicon DAA (Data Access Arrangement) circuit capable of monitoring G3 communications executed via the ISDN.
2. Discussion of Related Art
A conventional electric circuit forming a central core of a facsimile apparatus can generally be categorized into an FCU (facsimile control unit) and an NCU (network control unit) as shown in FIG. 3.
The FCU 101 includes a controller section 102 which controls various instruments built in a facsimile and a modem section which applies modulation/demodulation to a facsimile signal. The modem section typically includes a modem DSP (Digital Signal Processor) 103 performing digital signal processing. Also included in the modem section is an AFE (Analog Front End) section 104 having a CODEC (Compression/Decompression) which performs AD/DA conversions and an operational amplifier which adjusts a facsimile signal to a specified level.
The NCU 105 interfaces with a telephone line and is formed from an insulation transformer 106 which insulates the telephone line 109 (hereinafter referred to as a primary side) from the FCU 101 (hereinafter referred to as a secondary side), a direct current circuit 107 which meets a direct current closure standard for the telephone line, and a ring detection circuit 108 which detects a ring signal of the telephone line and so on.
As a facsimile communications system, widely spreading G (Group) 3 communications and high-speed G4 digital communications operating via an ISDN line are exemplified. According to the G3 communications system, a transmission modem temporarily converts a digital signal into an analog signal and then transmits the analog signal through a telephone line, and a reception modem converts the analog signal into digital data. In contrast, the G4 communications system transmits a digital signal as is via the ISDN line.
Such communications are performed without any problem between communication systems of the same type. However, due to a difference in a type of a communications system, a G3 facsimile apparatus and G4 a facsimile apparatus cannot communicate with each other unless one or both of them are adapted for such communication. For example, a facsimile apparatus having a function of G4 communications can include a G3 communications function in order to communicate with a facsimile apparatus operating in the G3 system. In such a situation, however, although a user installs and connects the IDSN line to his or her G4 facsimile apparatus, a communications counterpart is limited to a G3 facsimile apparatus when the G4 facsimile apparatus performs communications in a G3 mode via the ISDN line as shown in FIG. 4.
Specifically, facsimile signals input and output to and from the modem DSP 205 undergo AD/DA conversion in the CODEC0 section 206. When communications are performed via a conventional PSTN (Public Switched Telephone Network), input and output terminals of the CODEC0 section 206 are connected to the NCU and the PSTN line via an insulation transformer. Input and output signals to and from the CODEC0 section 206 are mixed in the addition amplifier 209 and are output from a speaker 207 as a mixture signal generating a sound. Thus, a facsimile signal on the PSTN line can be aurally monitored.
On the other hand, when facsimile communications are performed via the ISDN line, an SW1 section 208 switches a connection destination of the input and output terminals of the CODEC0 section 206 from the NCU to the CODEC1 section 202. The CODEC1 section 202 converts a facsimile communications signal transmitted from the CODEC0 section 206 into a digital signal of 8 bits having undergone μ/Alaw or μ/law conversion communicable at 8000 Hz sampling frequencies for the ISDN line. Such digital signal is then launched into the ISDN line via an ISDN interface section 203. Similar to the PSTN line connection, a facsimile signal on the ISDN line can be aurally monitored from the speaker 207.
It is increasingly the case recently that in such an environment, data modems employ a low cost and downsized silicon DAA (Data Access Arrangement) instead of using the above-mentioned transformer. With such a configuration, a problem which likely occurs in such a facsimile is that the G3 facsimile communications cannot successfully be performed via the ISDN. A countermeasure against the problem has been developed such that digital data subjected to sampling rate conversion to 8000 Hz and μ/Alaw or μ/law conversion is used to communicate between the data modem and the ISDN line.
Further, as an insulation device insulating the FCU from the telephone (PSTN) line, an NCU employing an insulation transformer is conventionally adopted. It is increasingly common, however, that data modem apparatuses employ a silicon DAA having an insulation device such as a condenser. Thus, it is more common now that a facsimile apparatus will use a silicon DAA as shown in FIG. 5.
Specifically, the silicon DAA utilizes such an insulation condition of a condenser and is typically formed from a system side device 305 and a line side device 307 coupled through the insulation condenser 310. In such a facsimile apparatus, a facsimile communications signal from the modem DSP section 304 is converted into a signal capable of passing through the insulation condenser 310 from the system side device 305, to the line side device 307. The line side device 307 converts the signal from the modem DSP 304 with a built-in CODEC section 3071 and outputs an analog signal to a line. In contrast, an input signal received via the PSTN line undergoes A/D conversion in the built-in CODEC section 3071, and is transmitted to the system side device 305 via the insulation condenser 310, which is ultimately input to the modem DSP 304.
However, when facsimile communications are performed via the ISDN line from the conventional system of FIG. 3 employing the silicon DAA, the ISDN communications block of FIG. 4 must be connected to the system. As a result, the input to and output from the ISDN communications block are enough to directly communicate with the CODEC1 section 202, and accordingly, the system of the silicon DAA has no provision (i.e., data communication route) to transmit a facsimile communications signal to a system side device 305 of the secondary side connecting to a speaker.
Further, due to an 8 bits and μ/Alaw or μ/law conversion receiving digital signal communicable at 8000 Hz through the ISDN line, the above-mentioned facsimile communications signal cannot be monitored through the speaker if such a format is maintained. That is, these inputs to and outputs from the modem DSP 304 and the system side device 305 are entirely digital signal states, and there are no analog signals for driving the speaker.